Twin probe waveguide transition



Oct, 20, 1959 J. HESSLER, JR

TWIN PROBE WAVEGUIDE TRANSITION Filed Dec. 8, 1955 A P Y INVENTOR.

m My 2 E R w M W T N w W/d J n ed States Patent PROBE WAVEGUIDE TRANSITION .l'ohn Hessler, In, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation This invention relates to a coupling arrangement between a rectangular waveguide and two coaxial cables, and is particularly directed to means for determining the optimum distance between the cables and their placement with respect to the closed end of the waveguide.

It is known that the energy in a waveguide maybe transmitted to or received from a single coaxial cable with the inner conductor extending probe-fashion into the waveguide. Mechanical and electrical factors have made it desirable to provide a transition between a single rectangular waveguide and two coaxial cables.

The object of this invention is an improved structure for coupling a single rectangular waveguide with two coaxial cables in which the important dimensions are properly chosen so that the various impdances properly match, and there is a minimum of energy reflections in any direction.

The object of this invention is attained by extending the inner conductors of the coaxial cables into the waveguide probe-fashion and matching the impedance in the waveguide to the cables while the cables are terminated with impedances equal, respectively, to the characteristic impedances of the cables.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a sectional view, taken on the line 1-1 of Fig. 2, of one embodiment;

Fig. 2 is a sectional view, taken on the line 2-2 of Fig. l, of the waveguide and coaxial cables of Fig. 1;

Fig. 3 is a sectional view of another embodiment; and

Fig. 4 shows in section an alternative cable-to-wave guide coupling detail.

The waveguide 1 contemplated here is rectangular in coss-section and is broader in one dimension than in the other. Commonly the height B of the side 'walls 2 and 3 is about .45 times the width A of the top and bottom panels 4 and 5. Coaxial cables 6 and 7 each comprise an outer tubular conductor 8 and 9 and a coaxial inner conductor 10 and 11 spaced apart by sleeves .12 and 13 of low-loss insulating material of the thermo plastic type. The outer conductors are sealed over openings inthe top 4 of the waveguide as by collars 14 and 15 into which the tubes 8 and 9 may be inserted and soldered. Alternatively, the tubes may be held in place by the threaded collar assembly as shown in Fig. 4. By

*flaring the end of the tube 8, or by soldering on a ferrule, a threaded collar will .tighten the tube to a bushing 21 set into the wall of the waveguide.

The inner conductors extend into the waveguide disstance P, the insulating sleeve also extending into the waveguide and terminating short of the ends of the conductors 10 and 11. To prevent movement of the insulating sleeves 12 and 13 as the temperature changes, and

2,909,735 Patented Oct. 20, 1959 to thus prevent changes in the electrical characteristics to the probe formed by the inner ends of the conductors, caps 16 and 17 are soldered to the ends of conductors 10 and 11 snugly against the ends of the sleeves. Caps 16 and 17 also have the additional advantage of improving the manufacturing tolerance of dimension P.

The two coaxial cables are placed side-by-side and equal distances from the terminating plate 18 across the end of the waveguide. The distance L from the end of the waveguide to the coaxial cables may be adjusted by moving the end plate telescopically in the end of the waveguide. The distance D from the center of the waveguide to each cable is equal and the magnitudeof dimesion'D is a function of dimensions L and P and of the operating frequency. Further, the dimensions are important functions of the width of the band of frequencies to be transmitted between the waveguide and cables.

In designing the structure of Fig. 1, first a single probe transition is employed and the optimum distance L for that single probe is calculated for the band of frequencies.

Next, the type. of probe is considered which will yield the least critical probe length. The size of the inner conductors 10 and 11 and caps 16 and 17 are selected to cause the probe to have least critical length.

Next, the off-center distance D is calculated by the approximate formula:

where rm equals the minimum voltage standing wave ratio for a single probe at distance L from the end of the waveguide, the minimum VSWR being obtained by varying the probe penetration, P, through several increments, and where A is the major dimension of the waveguide. Then the two probes are inserted; each is the distance Dfrom the center.

Then with both coaxial lines terminated, the admittance of the junction is measured through the Waveguide for'various probe lengths and the admittances are plotted on a SmithHtype chart with the reference point at the probes. If the susceptance is positive, the off-center-probe distance D should be increased, but if the susceptance is negative the distance D should be decreased. The distance D for frequencies in the X-band has been found to be in the range of .215". The impedance match for any given design is obtained by adjusting the probe length to match the resistive component of the energy transmitted and by adjusting distances D and L to match the reactive component of the probe as measured in the waveguide with the coaxial lines terminated. If the dimension D/A should be greater than /3, it is possible that the manufacturing tolerance in D will be too critical. Then, D/A should be reduced to a value of less than /3 and P and L varied for an impedance match.

It has been found by extensive computation, substantiated by experimentation, that the distance D in terms of waveguide width A, that is, D/A, should be between about A1, and /3. The minimum value of D/A is limited by the band width to be transmitted, and by mechanical considerations. The smaller D/A is, the shorter L will be, thereby causing a narrower band width to be transmitted. The maximum practical value of D/A, however, is fixed by permissible manufacturing tolerances, as suggested above. The greater dimension 'D is, the more critical will be the manufacturing tolerance of D.

The distance L in terms of the waveguide length, that is, L/n has been found, in the same experimentations, to be variable between /6 and A. If this ratio becomes less than /6, the band width for a good impedance match becomes critically narrow, while if the ratio of A is exceeded, the manufacturing tolerance of the D dimension becomes too difficult because D/A will become large. The coaxial cable actually used in these experimentations was of a commercially available type, with a characteristic impedance of 50 ohms, the frequencies employed were in the region of 7,000 to 10,000 megacycles per second, and the waveguide dimensions were A:.9 inch, and B:.4 inch.

Specifically, in one factory production model D/A was established at .239, L/n was fixed at .165, and P/B was found to be .40 for optimum manufacturing tolerances. In practice, after the D and the L dimensions were determined, the probe length P was adjusted for optimum impedance match with the coaxial lines terminated. Where the band to be transmitted must be fairly broad, the D/A ratio should be increased to about .322 and the L/n ratio should be increased to about .222.

Where, for mechanical reasons, it is desired that the two coaxial cables extend in opposite directions from the waveguide, the cables may be inserted into opposite broad sides of the waveguide, as shown in Fig. 3. Surprisingly and fortunately, the probe penetration P, spacings D between cables, and spacing L from the end plate 18 remain substantially the same as in Fig. 1.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. In combination, a rectangular waveguide with a closed end and two coaxial cables, one end of each outer conductor of the cables being joined to an opening in a broad side of the waveguide of width A, and the inner conductors of the cables extending into the waveguide, the cables being equal distances D from the center plane of the waveguide and each being distance L from the closed end of the waveguide, the ratio D/A being between about and about /3, and the distance L being between about /6 and about A of the operating waveguide wavelength, a sleeve of low-loss insulating material separating the inner and outer conductors of the coaxial cables respectively, the sleeves extending into the waveguide and terminating short of the ends of the inner conductors, and

metal caps secured to the ends of the inner conductors snugly against the ends of the insulating sleeves, thereof preventing movement of said sleeves due to temperature changes.

2. In combination, a rectangular waveguide with a closed end and two coaxial cables, one end of each outer conductor of the cables being joined to an opening in a broad side of the waveguide of width A, and the inner conductors of the cables extending into the waveguide, the cables being equal distances D from the center plane of the waveguide and each being distance L from the closed end of the waveguide, the ratio D/A being between about A and about /3, and the distance L being between about /6 and about of the operating waveguide wavelength, said inner conductors extending through opposite broad sides, respectively, of the waveguide.

3. In combination, a rectangular waveguide with a closed end and two coaxial cables, one end of each outer conductor of the cables being joined to an opening in a broad side of the waveguide of width A, and the inner conductors of the cables extending into the waveguide, the cables being equal distances D from the center plane of the Waveguide and each being distance L from the closed end of the waveguide, the ratio D/A being between about and about /a, and the distance L being between about /6 and about A of the operating waveguide wavelength, said inner conductors extending through opposite broad sides, respectively, of the waveguide, a sleeve of low-loss insulating material separating the inner and outer conductors of the coaxial cables respectively, the sleeves extending into the waveguide and terminating short of the ends of the inner conductors, and metal caps secured to the ends of the inner conductors snugly against the ends of the insulating sleeves, thereby preventing movement of said sleeves due to temperature changes.

References Cited in the file of this patent UNITED STATES PATENTS 2,443,654 Else et a1. June 22, 1948 2,527,146 Mumford Oct. 24, 1950 2,589,843 Montgomery Mar. 18, 1952 

